1. Field of the Invention
This invention relates to a magnetoinductive flowmeter for measuring the flow rate of a moving medium, with a measuring conduit through whose wall extends a sampling-electrode channel, and a sampling electrode that is positioned in the sampling-electrode channel in such fashion that the electrode head is recessed from the inside wall surface of the measuring conduit.
2. Description of the Prior Art
Magnetoinductive flowmeters have essentially been well-known for some time and have been employed in a variety of applications. The basic principle of a magnetoinductive flowmeter for measuring the flow rate of fluids and liquids goes all the way back to Faraday who, as early as 1832, suggested utilizing the principle of electrodynamic induction for measuring flow rates. According to Faraday's law of induction, a flowing medium that contains charge carriers and travels through a magnetic field will generate an electric field intensity perpendicular to the flow direction and perpendicular to the magnetic field. In a magnetoinductive flowmeter, Faraday's law of induction is employed in that a magnet, usually consisting of two field coils, generates a magnetic field perpendicular to the direction of flow through the measuring tube. Within that magnetic field, each volume component of the flowing medium, containing a given number of charge carriers, contributes its portion of the field intensity generated therein to a measuring voltage that can be collected via measuring or sampling electrodes. In conventional magnetoinductive flowmeters, the sampling electrodes are designed for conductive or capacitive coupling with the flowing medium. One salient feature of magnetoinductive flowmeters is the proportionality between the measured voltage and the flow rate of the medium as averaged across the diameter of the measuring tube, i.e. between the measured voltage and the volumetric flow.
A magnetoinductive flowmeter as referred to above is described for instance in WO 85/04954. In that particular magnetoinductive flowmeter, the section of the sampling electrode channel located in front of the sampling-electrode head and extending up to the interior of the measuring conduit is provided with a cap consisting of a porous ceramic material, of a glass frit or of a porous synthetic material. That cap is intended to prevent any fouling of, or other interference with, the surface of the sampling-electrode head or erroneous measuring results due to solid particles carried along by the flowing medium. While the porous cap protects the surface of the sampling-electrode head, it allows the liquid medium to penetrate so as to establish a conductive connection between the flowing medium and the sampling electrode. The end surface of the cap itself is mounted flush with the inner wall surface of the measuring conduit, which protects it from any impact by the flowing medium and from any abrasion by hard, solid particles carried along by the medium. Evidently, the porosity of the cap in front of the sampling electrode channel must be so chosen that it is permeable to liquids yet prevents solid particles in the medium from penetrating to the surface of the sampling-electrode head.
These solutions according to WO 85/04954 and other prior-art notwithstanding, a voltage signal collected at the sampling electrode will still contain a significant noise component especially when there is a substantial proportion of solid particles such as sand, etc. in the flowing medium. To be sure, the magnetoinductive flowmeter referred to above reduces that problem in that the sampling-electrode head itself is set back from the inner wall surface of the measuring conduit. However, given the fact that the cap, consisting of a porous material such as porous ceramic, fills the space in the sampling electrode channel in front of the sampling-electrode head, the sampling electrode receives only a weak signal, which by itself has a detrimental effect on the signal-to-noise ratio.